The invention relates to a device for controlling an electronically commutated motor comprising P phases and N pairs of poles, a method for using such a device and an anti-friction bearing integrating the encoder and sensor of such a device.
On account of their high specific power, electronically commutated motors, or brushless motors, can be used to control the rotation of a mechanical member in a large number of applications.
One particular example of such an application is power-assisted steering of motor vehicles by an electric power-assisted steering system.
Within the scope of its application, it is necessary to control precisely the commutation of the current in the phases of these motors in order to obtain a torque which is optimum and equal to a set point with the fewest modulations possible.
To do this, use is known of a sensor comprising three sensing elements disposed opposite an encoder having N pairs of poles, said encoder being rotated by the motor.
In the case of a three-phase motor, by providing that the three sensing elements are phase-displaced with respect to one another by a mechanical angle making it possible to supply three electrical signals phase-displaced by 120 electrical degrees, it is possible to control the commutation between the phases of the motor at the right moment.
But this type of implementation can cause modulations of the motor torque which are due in particular to:
the reluctance torque of the motor;
faults of commutation of the current from one phase to another depending on the speed of rotation of the motor;
the absence of overlapping of the trapezoidal EMFs.
There is known, in particular from the document FR-2 749 452, a device for controlling an electronically commutated electric motor which makes it possible to partially avoid the torque faults related to the commutation of the current from one phase to another.
But, in order to avoid the torque modulation related to the motor design (reluctance torque and absence of overlapping of the trapezoidal EMFs), it then becomes necessary to know to a fine degree the absolute position of the rotor in order to control the current in the phases according to the position of the rotor.
Such a control strategy therefore necessitates an additional sensor giving a high-resolution position in addition to the one controlling the commutation.
This type of solution, in providing two sensors, leads to significant integration constraints and a non-optimised cost. Moreover, it can be penalising in terms of mechanical inertia of the rotor.
The invention aims in particular to resolve these drawbacks by proposing a commutation device comprising a two-track encoder movable rotation-wise and a single sensor fixed rotation-wise which is capable of delivering on the one hand signals for commutating the current in the phases and on the other hand high-resolution position signals allowing the control of the current supplying the phases, with a sufficiently fine resolution to avoid motor torque modulations.
To that end, and according to a first aspect, the invention proposes a device for controlling an electronically commutated motor comprising P phases and N pairs of poles, said device comprising:
a circular multipole encoder intended to perform, together with the rotor of the motor, a plurality of revolutions around its axis of rotation, said encoder comprising a main multipole track and a multipole commutation track comprising 2*P*N singularities equidistributed angularly, the main and commutation tracks being concentric on the encoder;
a fixed sensor disposed opposite and at air gap distance from the encoder, comprising at least three sensing elements, at least two of which are positioned opposite the main track so as to deliver two periodic electrical signals S1, S2 in quadrature, and at least one of which is positioned opposite the commutation track so as to deliver an electrical signal S3, the sensor comprising an electronic circuit capable, from the signals S1, S2 and S3, of delivering square digital signals A, B in quadrature and a digital signal C in the form of 2*P*N pulses per revolution of the encoder;
a circuit for commutating the currents in the phase windings of the motor having 2*P*N switches;
a control circuit for the commutation circuit which:
from the signal C supplies commutation signals for the switches; and
from the signals A, B and according to a first current set point defines a second current set point which controls the current supplying the phase windings of the motor.
According to a second aspect, the invention proposes a method for using such a control device in which the control signals make it possible, according to predetermined commutation logic, to actuate the switches in twos in turn, the transition from the actuation of two switches to the following two taking place each time an electrical pulse of the signal C is detected.
According to a third aspect, the invention proposes an anti-friction bearing intended to allow the rotation of the rotor of an electronically commutated motor comprising P phases and N pairs of poles or that of a member rotated by said rotor, said anti-friction bearing comprising a fixed ring, a rotating ring and rolling bodies disposed between said rings, in which:
there is associated with the rotating ring a circular multipole encoder comprising a main multipole track and a multipole commutation track comprising 2*P*N singularities equidistributed angularly, the main and commutation tracks being concentric on the encoder; and
there is associated with the fixed ring a fixed sensor disposed opposite and at air gap distance from the encoder, comprising at least three sensing elements, at least two of which are positioned opposite the main track so as to deliver two periodic electrical signals A, B in quadrature, and at least one of which is positioned opposite the commutation track so as to deliver an electrical signal C in the form of 2*P*N electrical pulses per revolution of the encoder, the signal C being intended to supply commutation signals to the motor and the signals A, B being intended, according to a first current set point, to control the current supplying the motor.